The present invention relates to a liquid separation module of a spiral type employing semipermeable membrane sheets for separating a desired liquid component from a feed solution, and utilizing the principle of reverse osmosis or ultra-filtering and apparatus for producing such a module.
A liquid separation module is of complicated construction. Normally, at least two semipermeable membrane sheets, as well as one kind of spacing layer for a first passage for a permeated solution and another kind of spacing layer for a second passage for a feed solution are spirally wound about a hollow mandrel having at least one hole. The first passage is formed between the inside surfaces of the two membrane sheets and communicates with the interior of the hollow mandrel through the hole, and the second passage is formed between the outside surfaces of the two membrane sheets.
The specifications of U.S. Pat. No. 3,386,583, U.S. Pat. No. 3,397,790, and O.S. W. Reports No. 879 and No. 341 (Office of Saline Water) or PB-228061 and PB-204107 disclose methods of producing liquid separation modules having a construction such as mentioned above. Each of these disclosed methods requires an apparatus of large dimensions and of very complicated construction, having a mechanism designed so that all of the layer elements to be wound are radially or tangentially tensioned separately and the tensioned layer elements are wound around a stationary hollow mandrel.
Generally speaking, to wind a plurality of layer elements, such as those of a liquid separation module, about a mandrel, it would be convenient to adopt a method wherein the layer elements, arranged one on top of the other, are spirally wound about a mandrel by rotating the mandrel with the forward end of the layer arrangement fixed thereto, while the layer arrangement as a whole is tensioned. However, with this method, the differences in curvatures of neighboring inner and outer layer elements in the layer arrangement wound about the mandrel cause a difference in the winding velocities of the layer elements. This results in the inner layer elements shifting rearwardly relative to the outer layer elements. Such shifting movement would occur smoothly and the inner layer elements would be shifted by an easily predetermined amount relative to the outer layer elements, if the layer arrangement were loosely wound and the neighboring layer elements were smoothly slidable on each other. However, the layer elements employed in a liquid separation module, that is, membrane sheets and spacing layers, are not smoothly slidable on each other. A liquid separation module is required to have tightly wound layer elements. One deterrent to smooth sliding within the layer arrangement employed in liquid separation modules is that the arrangement has wrinkles caused by the layer arrangement having a nonuniform thickness. This nonuniform thickness of the arrangement is due to the factors noted below.
A. In general, it is difficult to produce layer elements, particularly spacing layers, of uniform thickness. PA1 B. Piling different kinds of layer elements one on top of the other, each having a nonuniform thickness, enhances the nonuniform thickness of the layer arrangement. PA1 C. Each membrane sheet has adhesive material layers at its edge portions so that it can be bonded to the neighboring membrane sheet with a spacing layer disposed between the two membrane sheets after they are wound together. This adhesive layer results in enhancing the nonuniform thickness of the layer arrangement, caused by the problem mentioned in A above, if the membrane sheet is not placed accurately on the spacing layer at a predetermined relative position with the result that the adhesive layer abuts the edge portion of the spacing layer. However, in practice, it is difficult to place the membrane sheet on the spacing layer at the predetermined relative position.
Consequently, if the above-mentioned seemingly convenient method were to be adopted for producing a liquid separation module under the condition that the layer arrangement is wound tightly about a mandrel, the inner layer element would shift rearwardly relative to the outer layer element by an amount less than the before-mentioned predetermined amount, which would cause the occurrence of many wrinkles along its length and in the other layer elements. The wrinkles in the layer elements, particularly in the membrane sheets, damage the permeating function of the sheet, with the result that the performance of the liquid separation module is reduced.
Further, if the above-mentioned seemingly convenient method were applied for making a liquid separation module, the wound layer arrangement would not only have the above disadvantages but also the following disadvantage. In general, the inner portion of such a spirally wound layer arrangement tends to be loosely compared with the outer portion thereof. However, in a liquid separation module utilizing the principle of reverse osmosis, such a nonuniform tightness results in an unequally distributed fluid flow in the permeated solution passage and feed solution passage. This, in turn, produces the phenomenon of polarized concentration, which decreases rejection, i.e. the performance of the module, and further, leads to such deformation of the wound layer arrangement as telescoping.
Conventional methods employ a mechanism for separating portions of layer elements which have not yet been wound from each other the length thereof, and for tensioning the layer elements separately while they are being wound. This prevents damage to the wound layer elements by such abnormal relative shifting with wrinkles, as mentioned above, and permits uniform tightness of winding along the radius of the spirally wound layer arrangement to be obtained. However, an apparatus provided with such a separating and tensioning mechanism requires not only a complicated construction but also troublesome manual operations, by which all of the layer elements are individually mounted to the apparatus. Further, it requires delicate tension control for tensioning the separate layer elements to thereby ensure uniform tightness to a predetermined degree. In addition, in conventional apparatus, wherein the layer elements are being tensioned separately and wound, if the adhesive material were coated on the entire edge portions of the membrane sheets to bind the neighboring membrane sheets, it would have a tendency to flow away from and/or along the edge portions thereof. Therefore, in practice, such adhesive material is, for example, coated on parts of the edge portions just before said parts reach the mandrel. However, such a partial coating operation is very troublesome.